After its successful launch in December, the European Space Agency’s (ESA) Gaia has now taken up its position in orbit and is ready to survey the skies. With the help of two onboard telescopes focused onto the largest-ever camera sent to space, the space observatory is expected to catalog nearly one billion stars throughout its five-year mission.

ESA’s Gaia will map stars in the Milky Way. It will do this by measuring the brightest billion objects and determining their three-dimensional distribution and velocities. It also has the ability to measure the temperature, mass, and chemical composition of each of the objects.

The brightest objects won’t necessarily need to be very bright in order to be included in the catalog. Gaia will be able to discern objects up to 400,000 times dimmer than those visible to the naked eye. And the positional accuracy of its measurements are akin to measuring the width of a human hair at a distance of 500 km.

The survey process will involve scanning each part of the sky an average of 70 times over the course of the five-year mission, which means scanning the entire sky twice every 63 days, once through each of the two telescopes. The repeated imaging makes it a powerful tool for spotting objects that evolve over time, such as binary systems, supernovae, and exoplanets.

This isn’t the first time that the ESA has done a grand galactic survey; its Hipparcos mission did this in the early 1990s. But compared to Hipparcos, Gaia will be able to measure 500 times the number of stars and extend its imaging to objects 1,000 times dimmer than Hipparcos could catalog.

The technology that makes this possible is the largest camera ever launched into space—sporting 940 million pixels. A lot of effort before launch was on figuring out exactly how to get the huge amount of data Gaia will produce back down to Earth.

When a picture is taken, a number of charged-coupled devices (CCDs)—sensitive imaging sensors—are used to spot objects before they are visible in the main focal plane. This allows the instrument to track the objects as they pass across the camera and only retain data from small regions around the object, reducing the amount of information that needs to be sent to Earth. In five years, Gaia will send only 100 TB of data. Once the data arrives to Earth, there is a system in place to immediately analyze it and distribute alerts to ground-based observatories if anything quickly evolving and potentially interesting is spotted, such as a supernova.

The catalog produced by Gaia is expected to contribute to many areas of astrophysics and to multiply our database of exotic objects, such as exoplanets, white and brown dwarfs, and supernovae many-fold. It will also contribute to more precise measurements of General Relativity, help to constrain the measurements of the presence and location of dark matter, and give us more accurate information about our galactic neighborhood and its evolution.

Gaia was successfully launched on December 19. After a month’s transit, it is now in orbit at about 1.5 million km away from Earth. By virtue of its position opposite the sun from the Earth and its large sun shield, it will be able to see objects at 45 degrees from the sun, allowing it to spot asteroids with orbits that lie between the Earth and the sun. These are candidates for Earth collision and very difficult to observe from the ground.

Staff on the ground is conducting in-orbit testing, during which the exact orbital parameters are determined, and all systems are tested for performance. Calibration images have been obtained, and the ground team is working on procedures to resolve a few remaining issues, such as reducing contamination on the CCDs and dealing with sunlight diffracted around the sun shield.

Beyond items flagged by the alert system for ground-based follow up, it will take two years to complete the first proper Gaia catalog and make it available to the wider scientific community. Following this, new iterations will be issued about once a year; these will focus on precision measurements of the characteristics of the objects in the Gaia catalog. It is expected that Gaia’s database will have many new objects and phenomena hidden within it, and mining the data will keep astronomers busy for decades to come.

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